Present Status of Aircraft Instruments Page: 3 of 22
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PRESENT STATUS OF AIRCRAFT INSTRUM- ,NTS
ment at the Bureau of Standards, using the commuta-
tor-condenser principle of the suspended-head air-
speed meters now in use on lighter-than-air craft.
(References 1 and 2.)
True air-speed rmeter.--Air-speed instruments with
rotating surfaces, such as Robinson cups or a propeller,
give readings independent of the density of the air,
and are known as true air-speed meters. Instruments
of this type are preferred on airships because they are
especially suited for the measurement of air speeds
in the lower speed ranges.
An instrument used by the United States Navy on
airships is that known as the commutator-condenser
type, in which a condenser is alternately charged and
discharged, in the latter case through an ammeter,
the rate being governed by the propeller trailing in
the air. The number of discharges per second deter-
mines the reading of the ammeter, which can thus be
calibrated in terms of air speed. (Reference L)
When a knowledge of true air speed is required in
addition to the indicated air speed, attention should be
directed to the possibility of obtaining this data with
one instrument. The true and indicated air speed
differ by a factor depending on the air density. The
latter may be assumed with sufficient accuracy in most
cases to be that of the standard atmosphere at the indi-
cated altitude. The Pitot-static tube instrument may
consequently be provided with a manually-operated
adjustment in the mechanism so that either true or
indicated air speed can be indicated at wil The ad-
justment serves to control the multiplication of the
mechanism of the indicator. An experimental model
of an instrument of this kind has been developed for
the Bureau of Aeronautics of the Navy by the Bureau
of Standards, which shows the idea to be practicable.
Ground speed.-In ground-speed measurements, as
carried out at present, it is necessary that the ground
shall be visible. The methods used (in the few cases
where ground speed is measured) follow:
(a) The wind-star method.
(b) Traverse method.
(c) Optical method.
(a) The wind-star method of measuring ground
speed has been adopted as standard by the Bureau
of Aeronautics of the Navy. This method does not
require a knowledge of the tapeline altitude, but it
necessitates flying widely off course for a short time,
or in the modified form, flying at two speeds. This
method is part of the procedure for the determination
of the heading of the aircraft and is therefore de-
scribed under Navigation Instruments. It requires
a minimum of apparatus and a relatively small
amount of computation, provided the course heading(b) The traverse method is very often used in meas-
uring ground speed. The traverse instrument con-
sists of two parallel wires attached to the aircraft,
together with a rotatable sighting telescope or eye-
piece. The correction for altitude is made by a linear
motion of either the eyepiece (or telescope) or one
of the wires. An object is sighted under one of the
wires and the time required for it to come under
the second wire is measured. The time interval, the
constants of the apparatus, and the altitude are suffi-
cient to determine the ground speed. The method is
dependent on a knowledge of the tapeline altitude
above the ground, which constitutes a serious limita-
tion in its usefulness.
(c) The optical metho depends on neutralizing the
apparent speed of some object on the ground by means
of a rotating reflector or refractor. The rate of rota-
tion of the reflector or refractor is proportional to
the ratio of the ground speed to the altitude of the
airplane above the observed object. An instrument
of the refractor type suggested by Anderson was built
for the United States Army Air Corps by the Bureau
of Standards about six years ago. From this experi-
ence it appears that an instrument either of the re-
flector or the refractor type can be made mechanically
satisfactory and at the same time can be so designed
as to require no computation by the pilot. This
method is also dependent on the tapeline altitude. An
error of 5 per cent in the altitude measurement intro-
duces an equal error in the ground-speed determina-
tion.
Ground-speed instruments which do not depend on
the visability of the ground have been proposed. (See
Reference 5.) There has been no recent progress in
their development.
2. ALTITUDE INSTRUMENTS
This class of instruments includes the following:
Aneroid altimeter.
Barograph.
Absolute altimeter.
Rate-of-climb meter.
Free-air thermometers.
The latter instrument is included here for want
of a better classification.
Altimeter, aneroid type.-The performance of this
type of altimeter has been improved remarkably in
the last few years. The accuracy of the best com-
merciaI instruments (quantity production) is now
superior to that of any instrument on the market in
1926. The usefulness of altimeters now appears to
be limited by the inherent defects of the barometric
method of measuring altitude rather than by instru-
mental difficulties. As recently as six or seven yearsis determined at the same time.
155
ago, reports were common that an altimeter set to zero
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Subcommittee on Instruments. Present Status of Aircraft Instruments, report, October 9, 1930; (https://digital.library.unt.edu/ark:/67531/metadc66026/m1/3/?q=led+zeppelin: accessed June 6, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.